1. Field of the Invention
The present invention relates to a wirecut electrical discharge machine (EDM) and more particularly to a wire guide apparatus which reduces the wear of a feeder therein for feeding a wire electrode and allows the feeder to be easily replaced.
2. Description of the Background Art
FIG. 14 illustrates the key sections of a known wirecut electrical discharge machine, which is provided with a wire electrode 1 (hereinafter referred to as the "wire"), a workpiece 2, a wire supply bobbin 3, a wire take-up bobbin 4, a wire tension controlling electromagnetic brake 5, a wire running speed control motor 6, an upper wire roller 7, an upper wire guide 8, a lower, wire guide 9, a lower wire roller 10, and an electrical discharge machining power supply 11 for supplying a pulse voltage across the wire 1 and workpiece 2 as machining energy. A voltage detector 12 is coupled to machining power supply 11, the wire 1 and the workpiece 2 for comparing an actual gap voltage between the wire 1 and workpiece 2 with the output voltage of the machining power supply 11. A numerical control unit 13 controls the relative motion of the wire 1 with respect to the workpiece 2 and processes the output signals of the detector 12.
The lower wire guide 9 will now be described in detail with reference to FIGS. 15 and 16. FIG. 15 is a sectional view illustrating a wire guide similar to that disclosed in Japanese Patent Disclosure Publication No. 249531 or U.S. Pat. No. 4,896,013. FIG. 16 is a detailed sectional view of an electrode guide. A hexagonal feeder 21 made of material such as carbide alloy, tungsten or silver tungsten, and an electrode guide 22 embedded with a die guide 23 made of diamond or sapphire, or the like, are utilized for guiding the wire 1.
Usually, the electrode guide 22 is also provided above the workpiece 2 as the upper wire guide 8 (see FIG. 14) to guide the wire 1. As shown in FIG. 17, the electrode guide 22 is provided with a small hole 24. Also, several dielectric grooves 25 are provided in the electrode guide 22. A holder 26 accommodates a feeder 21 (see FIG. 15) and inside the holder 26 is provided a hexagonal hole identical to the external shape of the feeder 21 so that the inner hole of the feeder 21 is eccentric with respect to the center of the electrode guide 22. An eccentric die 27 having a guide is fixedly provided below feeder 21 for securing the feeder 21 and bringing the wire 1 into contact with the feeder 21. A case 28 accommodates the holder 26 and a fixing screw (not shown) for securing the holder 26. A locating pin 30 is inserted in a hole formed in the holder 26 so as to insure proper positioning. A groove 31 is provided in the case 28 for maintaining the locating pin 30 in a given positional relationship with respect to case 28. A nozzle 32 for converging dielectric, and a stopper 33, secured to the case 28 for positioning the nozzle 32 in place with respect to the case 28, are also provided in the wire guide 9. 34 indicates a dielectric inlet.
The operation of the prior art apparatus constructed as described above will now be described with reference to FIG. 14, FIG. 15 and FIG. 16.
In wirecut electrical discharge machining, the workpiece 2 is electrically cut into a desired shape, by virtue of the numerical control unit 13 and a drive (not shown) which produces the relative motion of the wire 1 and workpiece 2, with a pulse voltage supplied as machining energy from the electrical discharge machining power supply 11 to a machining gap formed between the wire 1 and workpiece 2. The wire 1 is supplied from the wire supply bobbin 3, travels through the upper wire roller 7, wire tension controlling brake 5, upper wire guide 8, lower wire guide 9, lower wire roller 10 and wire running speed control motor 6, and is then retrieved by the wire take-up bobbin 4.
In the meantime, when a machining pulse voltage from the electrical discharge machining power supply 11 is applied to the wire 1 by the feeder 21 via the holder 26 an electrical discharge occurs between the wire 1 and workpiece 2. Accordingly, a machining pulse current flows through the feeder 21 and wire 1. Since the wire 1 is continuously supplied to the workpiece 2, heat generated by contact resistance between the feeder 21 and wire 1 or an electrical discharge phenomenon occurring due to any separation of the feeder 21 from the wire 1, causes the inner hole of the feeder 21 to wear gradually. This wearing erodes a contact state between the feeder 21 and the wire 1 and, eventually, leads to, a point where there is no contact between the feeder 21 and the wire 1 and therefore machining current cannot be properly fed to the wire. In such a case, the average output voltage of the machining power supply 11 at the detector 12 is raised by the stoppage of the current flow, whereas the voltage across the wire 1 and workpiece 2 is and the voltage detector 12 transmits a signal to the numerical control unit 13, which then stops machining or displays an alarm to change the feeder 21 on a CRT display, or the like (not shown). In such a case, the operator must remove the feeder 21, and manually change its orientation in holder 26.
During machining, some of the dielectric is supplied under pressure by a pump (not shown) into the inlet 34 and fed from the tip of the nozzle 32 to the machined area of the workpiece 2 through the dielectric grooves 25 formed in the electrode guide 22, thereby eliminating machining chips. Dielectric is also fed to the feeder 21 through the small hole 24 provided in the electrode guide 22, so as to suppress heat generated in the feeder section during machining.
To change the position of the worn feeder 21 in the known wirecut electrical discharge machine constructed as described above, it is necessary to stop machining, cut off the wire electrode 1, remove the eccentric die 27 and fixing screw 29, remove the feeder 21, change its angle to alter its contact area and then reassemble the device.
U.S. Pat. No. 4,945,200 discloses an embodiment wherein a feeder is rotated with respect to a wire supplying direction and an embodiment wherein a feeder is slanted. Although the devices disclosed by these references increase the life of the feeder, their arrangement is very complicated, resulting in high cost and difficult feeder replacement.
In the feeder unit disposed in Japanese Patent Disclosure Publication No. 221926, the contact area of a feeder and wire electrode is not cooled sufficiently, leading to a strong probability that the wire electrode will be broken in the feeder section. In addition, the structure of the feeder unit is very complicated which tends to increase its size.